Writing a blog about a topic you are not a complete expert in?

The term "expert" can be wide, but, what do you honestly think, how much "expertise" is necessary in a certain topic for you to run a blog or website about it?
How much previous knowledge do you need to have? Is it a good idea too learn a long the way, and do your research while you already start?
If you start a blog, for example, and start writing a certain article, but you don't know EVERYTHING about the topic and need to do some research to get enough quality for your posts